"Masters of Sex," which premieres Sunday on Showtime, takes as its subject Dr. William Masters and Virginia Johnson, the authors of "Human Sexual Response," "Human Sexual Inadequacy" and other medical potboilers of the 20th century. Without making any extraordinary claims for it, it is easy to watch and to recommend, mostly sweet-natured, with a host of well-shaded performances and almost nothing to insult your intelligence.

Created by Michelle Ashford ("The Pacific") from Thomas Maier's book of the same name, it is a story of men waking up to women and women waking up to themselves. (It begins in 1956, when, as the opening title card has it, "a nationally renowned fertility specialist met a former nightclub singer.") It is a biopic stretched into a series — not a miniseries, but a show to last possibly for years; its pace through their entwined lives and work is unhurried.

And although it hangs on bones of fact, it's more useful for the viewer to think of it as all made up. Because, mostly, it is, and because to the extent it tells the story of two real people, it also adorns the telling with dramatic practicalities, invented characters and narrative detours. Indeed, it's down these side streets, casting a brief light on a passing character (patients, prostitutes, provost's wife), that the show finds many of its best moments

It is also, as the story of a pair of professional voyeurs, a subject tailor-made for premium cable, which is always looking for ways to get people — women, mostly — out of their clothes and into a little simulated sex. Here, for once, it isn't gratuitous. Technically.

Michael Sheen stars as Masters. When we meet him, he is about to leave a dinner in his honor at St. Louis' Washington University, supposedly to deliver a baby. In fact, he is off to crouch in a closet in a brothel, with a stopwatch and a clipboard, to time a sex act.

He is a man with a mission. "The study of sex is the study of the beginning of all life," he declares, "and science holds the key! Yet we sit huddled in the dark like prudish cavemen, filled with shame. And guilt." He wants to study sex not as Kinsey had, just by asking questions, but by monitoring bodies in a laboratory.

"Only hookers and insane coeds would agree to this," someone objects, but it turns out some people just feel useful having sex for science.

Lizzy Caplan is Johnson, the "former nightclub singer," who comes to work at the hospital as a secretary and manages to attach herself to Masters and his project, eventually becoming his partner. As conceived here, she's the driving spirit, both within the story and of the series itself. Masters' journey, though it involves a kind of awakening (women fake orgasms?), some (narratively dubious) institutional strong-arming, domestic drama and a lot of science stuff, is simply not as dynamic as her self-reinvention.

Johnson is a new woman, by nature; she likes sex, which she doesn't confuse with love (in which she is rather less interested). She wants to do "something important," and in her head lives in a world where sisterhood is already powerful, a memo some of the women around her have yet to receive. She's the one who sees in their data on the clitoral orgasm "a brave new world" where a man is just a fish's bicycle.

Indeed, the series looks back on this time as the beginning of the (still-ending) end of male dominance and privilege. (One self-improving nurse, played by Heléne Yorke, is reading Simone de Beauvoir. "'The Second Sex'?" a flirty doctor asks. "Is that book as thought-provoking as it sounds?") The male characters do seem the thicker, more comical ones. But perhaps that is merely true.

Though we are made to see that Masters can be an understanding doctor dealing with a troubled patient, much of the time he is an arrogant, stiff-necked, workaholic pill, the risque nature of his chosen field notwithstanding. He is, I suppose, a sort of anti-hero — like his chronological near-contemporary Don Draper, emotionally stunted, work-defined, with hints of darkness in his past and a perfectly lovely wife (Caitlin FitzGerald) he treats respectably but also less than well.

She calls him "Daddy," which is meant both to describe their power relationship and to strike an ironic note, because they are trying to get pregnant, in a scientific way, and failing. (The Masters had two children, in fact, by the time the series begins.) Their perfect Modernist house is therefore not a home.

Beau Bridges, better employed here than on his new sitcom, "The Millers," plays Masters' friend and provost; Allison Janney, turning in some really lovely work, is his frustrated wife. Annaleigh Ashford is funny and touching as a sex worker looking to change her life; Margo Martindale pops in for a couple of dozen seconds to play an offended secretary; Barry Bostwick is a frisky pensioner.

It's a handsome thing, another well-dressed romp through the American mid-century, when things (we imagine) were simpler and (so we like to think) less sophisticated, but also more exciting. And it's true that sexual naiveté of that age can seem incredible in a day when pornography is just another thing on your platform of choice.

But even in an age when "Masters of Sex" is a TV show, the subject remains stubbornly powerful, private and confounding. We have come far, and we are still cavemen.